单选题I positively believe that there' s been no mistake.A. rationallyB. reasonablyC. certainlyD. boundly

单选题It is pleased for me to have met you here.A. a pityB. gladC. sadD. unwilling

单选题Greene spent a brief time at Cambridge.A. shortB. hardC. goodD. long

单选题 Plant Gas Scientists have been studying natural sources of methane for decades but hadn't regarded plants as a producer, notes Frank Keppler, a geochemist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Now Keppler and his colleagues find that plants, from grasses to trees, may also be sources of the greenhouse gas. This is really surprising, because most scientists assumed that methane production requires an oxygen-free environment. Previously, researchers had thought that it was impossible for plants to make significant amounts of the gas. They had assumed that, microbes need to be in environments without oxygen to produce methane. Methane is a greenhouse gas, like carbon dioxide. Gases such as methane and carbon dioxide trap heat in Earth's atmosphere and contribute to global warming. In its experiments, Keppler's team used sealed chambers that contained the same concentration of oxygen that Earth's atmosphere has. They measured the amounts of methane that were released by both living plants and dried plant material, such as fallen leaves. With the dried plants, the researchers took measurement at temperatures ranging from 30℃ to 70℃. At 30℃, they found, a gram of dried plant material released up to 3 nanograms of methane per hour. (One nanogram is a billionth of a gram.) With every 10-degree rise in temperature, the amount of methane released each hour roughly doubled. Living plants growing at their normal temperatures released as much as 370 nanograms of methane per gram of plant tissue per hour. Methane emissions tripled when living and dead plant was exposed to sunlight. Because there was plenty of oxygen available, it's unlikely that the types of bacteria that normally make methane were involved. Experiments on plants that were grown in water rather than soil also resulted in methane emissions. That's another strong sign that the gas came from the plants and not soil microbes. The new finding is an "interesting observation", says Jennifer Y. King, a biogeochemist at the University of Minnesota in St. Paul. Because some types of soil microbes consume methane, they may prevent plant-produced methane from reaching the atmosphere. Field tests will be needed to assess the plant's influence, she notes.

单选题Fine freshwater pearls are found in the main stream and the tributaries of the Mississippi River.______

单选题About one quarter of the workers in the country are employed in factories.

单选题______ they can do to help us is not known.A. whatB. whenC. thatD. where

单选题He paid the boy 10 dollars for washing ten windows, most of ______ hadn't been cleaned.A. theseB. thoseC. thatD. which

单选题The motorcar Some pessimistic experts feel that the automobile is bound to fall into disuse. They see a day in the not-too-distant future when all autos will be abandoned and allowed to rust. Other authorities, however, think the auto is here to stay. They hold that the car will remain a leading means of urban travel in the foreseeable future. The motorcar will undoubtedly change significantly over the next 30 years. It should become smaller, safer, and more economical, and should not be powered by the gasoline engine. The car of the future should be far more pollution-free than present types. Regardless of its power source, the auto in the future will still be the main problem in urban traffic congestion. One proposed solution to this problem is the automated highway system. When the auto enters the highway system, a retractable (可伸缩的) arm will drop from the auto and make contact with a rail, which is similar to those powering subway trains electrically. Once attached to the rail, the car will become electrically powered from the system, and control of the vehicle will pass to a central computer. The computer will then monitor all of the car's movements. The driver will use a telephone to dial instructions about his destination into the system. The computer will calculate the best route, and reserve space for the car all the way to the correct exit from the highway. The driver will then be free to relax and wait for the buzzer (蜂鸣器) that will waru him of his coming exit. It is estimated that an automated highway will be able to handle 10,000 vehicles per hour, compared with the 1,500 to 2,000 vehicles that can be carried by a present-day highway.

单选题{{B}}第三篇{{/B}} {{B}} Adaptation of Living Things{{/B}} Certain animals and plants develop characteristics that help them cope with their environment better than others of their kind. This natural biological process is called adaptation. Among the superior characteristics developed through adaptation are those that may help in getting food or shelter, in providing protection, and in producing and protecting, the young. That results in the evolution of more and more organisms that are better fitted to their environments. Each living thing is adapted to its way of life in a general way, but each is adapted especially to its own distinct class. A plant, for example, depends upon its roots to fix itself firmly and to absorb water and inorganic chemicals. It depends upon its green leaves for using the sun's energy to make food from inorganic chemicals. These are general adaptations, common to most plants. In addition, there are special adaptations that only certain kinds of plants have. Many animals have adaptations that help them escape from their enemies, Some insects are hidden by their body color or shape, and many look like steal or a little branch. The coats of deer are colored to mix with the surroundings. Many animals have the ability to remain completely still when an enemy is near. Organisms have a great variety of ways of adapting. 'They may adapt in their structure, function, and genetics; in their development and production of the young; and in other respects. An organism may create its own environment, as do warm-blooded mammals, which have the ,ability to adjust body heat exactly to maintain their ideal temperature' despite changing weather; Usually adaptations are an advantage, but sometimes an organism is so well adapted to a particular, environment that, if conditions change, it finds it difficult or impossible to readapt to the new conditions;

单选题Einstein's theory of relativity seemed incredible when it was first introduced.A. brilliantB. complicatedC. unbelievableD. distinguished

单选题Multivitamins Urged for All Pregnant Women A recent study in Tanzania found that when pregnant women took vitamins every day, fewer babies were born too small. Babies that weigh less than two and one-half kilograms at birth have a greater risk of dying. Those that survive are more likely to experience problems with their development. And experts say that as adults they have a higher risk of diseases including heart disease and diabetes. The World Health Organization (WHO) estimates that every year twenty million babies are born with low birth weight. Nine out of ten of them are born in developing countries. The new study took place in Dares Salaam. 4,200 pregnant women received multivitamins. The pills contained all of the vitamins in the B group along with vitamins C and E. They also contained several times more iron and folate than the levels advised for women in developed nations. Pregnant women especially in poor countries may find it difficult to get enough vitamins and minerals from the foods in their diet. The scientists compared the findings with results from a group of 4,000 women who did not receive the vitamins. A report by the scientists, from the United States and Tanzania, appeared in the New England Journal of Medicine . Wafaie Fawzi of the Harvard University School of Public Health led the study. None of the women in the study had HIV, the virus that causes AIDS. The scientists reported earlier that daily multivitamins were a low-cost way to reduce fetal deaths in pregnant women infected with HIV. The earlier work in Tanzania also found improvement in the mothers in their number of blood cells known as lmphocytes. Lymphocytes increase the body"s immunity against infection. The new study in pregnant women who were not infected with the AIDS virus found that multivitamins reduced the risk of low birth weight. Just under eight percent of the babies born to women who took the multivitamins weighed less than 2,500 grams. The rate was almost nine and one-half percent in the group of women who received a placebo, an inactive pill, instead of the vitamins. But the vitamins did not do much to reduce the rates of babies being born too early or dying while still a fetus. Still, the researchers say multivitamins should be considered for all pregnant women in developing countries.

单选题 Explorer of the Extreme Deep Oceans cover more than two-thirds of our planet. Yet, just a small fraction of the underwater world has been explored. Now, scientists at the Woods Hotel Oceanographic Institution (WHOI) in Massachusetts are building an underwater vehicle that will carry explorers as deep as 6,500 meters (21,320 feet). The new machine, known as a manned submersible or human-operated vehicle (HOV), will replace another one named Alvin which has an amazing record of discovery, playing a key role in various important and famous undersea expeditions. Alvin has been operating for 40 years but can go down only 4,500 meters (14,784 feet). It's about time for an upgrade, WHOI researchers say. Alvin was launched in 1964. Since then, Alvin has worked between 200 and 250 days a year, says Daniel Fornari, a marine geologist and director of the Deep Ocean Exploration Institute at WHOI. During its lifetime, Alvin has carried some 12,000 people on a total of more than 3,000 dives. A newer, better versions of Alvin is bound to reveal even more surprises about a world that is still full of mysteries, Fornari says. It might also make the job of exploration a little easier. "We take so much for granted on land," Fornari says. "We can walk around and see with our eyes how big things are. We can see colors, special arrangements." Size-wise, the new HOV will be similar to Alvin. It'll be about 37 feet long. The setting area inside will be a small sphere, about 8 feet wide, like Alvin, it'll carry a pilot and two passengers. It will be just as maneuverable. In most other ways, it will give passengers more opportunities to enjoy the view, for one thing. Alvin has only three windows, the new vehicle will have five, with more overlaps so that the passengers and the pilot can see the same thing. Alvin can go up and down at a rate of 30 meters every second, and its maximum speed is 2 knots (about 2.3 miles per hour), while the new vehicle will be able to ascend and descend at 44 meters per second. It'll reach speeds of 3 knots, or 3.5 miles per hour.

单选题You must try to wipe out the memory of these horrible events.

单选题Get Right Amount of Vitamin D Vitamin D helps bones and muscles grow strong and healthy. Low levels of vitamin D can lead to problems like rickets and osteoporosis. Rickets is a deformity mainly found in children. Osteoporosis is the thinning of bone, a common problem as people, especially women, get older. Studies in recent years have suggested that vitamin D may also have other uses. Studies have shown that low levels of D may increase the risk of heart attacks in men and deaths from some cancers. Other studies have shown that people with rheumatic diseases often have low levels of D. The easiest way to get vitamin D is from sunlight. The ultraviolet rays react with skin cells to produce the vitamin. But many people worry about skin cancer and skin damage from the sun. Also, darker skinned people produce less vitamin D than lighter skinned people. Production also decreases in older people and those living in northern areas that get less sunlight. Not many foods naturally contain vitamin D. Foods with high levels include oily fish such as salmon, tuna and mackerel, and fish liver oils. Small amounts of D are found in beef liver, cheese and egg yolks. Most of the D in the American diet comes from foods like milk with the vitamin added. These days, more doctors are testing for vitamin D levels in their patients. But as research continues, some experts worry that if people take too much D, it might act as a poison. Also, skin doctors warn people to be careful with sun exposure. How much vitamin D does a healthy person need? Nutrition experts who advise the American government set the current recommendations in nineteen ninety -seven. The daily amount is two hundred international units from birth through age fifty. Then it rises to four hundred I. U. s through age seventy, and six hundred for those seventy--one and older. But some groups say these amounts are not high enough.

单选题She felt that she had done her good Udeed/U for the day.

单选题 Harmless Revenge Revenge is one of those things that everyone enjoys. People don't like to talk about it, though. Just the same, there is nothing more satisfying or more rewarding than revenge. The purpose is not to harm your victims but to let them know that you are upset about something that they are doing to you. Careful plotting can provide you with relief from bothersome co-workers, gossiping friends, or nagging family members. Co-workers who make comments about the fact that you are always fifteen minutes late for work can be taken care of very simply. All you have to do is get up extra early one day. Before the sun comes up, drive to each coworker's house. Reach under the hood of your coworker's ear and disconnect the center wire that leads to the distributor cap, the ear will be unharmed, but it will not Start, and your friends at work will all be late for work on the same day. If you're lucky, your boss might notice that you are the only one there and will give you a raise. Gossiping friends at school are also perfect targets for a simple act of revenge. A way to trap either male or female friends is to leave phony messages on their lockers. If the friend that you want to get is male, leave a message that a certain girl would like him to stop by her house later that day. With any luck, her boyfriend will be there. The girl won't know what's going on, and the victim will be so embarrassed that he probably won't leave his home for a month. When Morn and Dad and your sisters and brothers really begin to annoy you, harmless revenge may be just the way to make them quiet down for a while. The dinner table is a likely place. Just before the meal begins, throw a handful of raisins into the food. Wait about five minutes and, after everyone has begun to eat, cover your mouth with your hand and begin to make odd noises. When they ask you what the matter is, point to a raisin and yell, "Bugs!" They dump their food in the disposal, jump into the car, and head for McDonald's. That night, you'll have your first quiet, peaceful meal in a long time. A well-planned revenge does not have to hurt anyone. The object is simply to let other people know that they are beginning to bother you.

单选题Under-used computer equipment can proliferate in large companies without a system to {{U}}monitor{{/U}} purchasing.

单选题The manager {{U}}allocates{{/U}} duties to the clerks. A.assign B.persuade C.ask D.order

单选题UEfficient/U air service has been made available through modern technology.